1. FIELD OF THE INVENTION
The present invention relates to the heat transfer labeling of designs onto an article, and in particular to decorative laminates to be used in the process.
2. Description of the Prior Art
Prior art methods for imprinting designs onto articles using a heat transfer labeling process typically involve decorative laminates consisting of a paper base sheet or web coated with a wax over which a design is imprinted in ink.
U.S. Pat. No. 3,616,015 is illustrative of the prior art. In U.S. Pat. No. 3,616,015 a label carrying web involving a heat transfer labeling process for imprinting designs onto articles is subjected to heat and the laminate is pressed onto an article with the ink design layer making direct contact with the article. As the paper sheet is subjected to heat, the wax layer begins to melt so that the paper sheet can be released from the wax layer. After transfer of the design to the article, the paper sheet is immediately removed, leaving the design firmly attached to the surface with the wax layer exposed to the environment. The exposed wax layer is then subjected to jets of hot air to remelt the wax which forms a clear protective coating over the ink design as it cools and solidifies. This prior art method has the advantage that the ink layer can be composed of a multiplicity of colors and the process may be reliably adapted to an automated process for imprinting designs onto a variety of articles including glass and plastic. However, the principal disadvantage of the method is that since the laminate is transferred directly to an article from a large continuous web, the laminate lacks sufficient flexibility to conform to surfaces having compound or sharp curvature. The method is therefore not effectively adaptable to imprinting objects having surfaces of compound or irregular curvature or recessed panels.
U.S. Pat. No. 3,616,176 discloses a heat transfer laminate of a type related to that disclosed in U.S. Pat. No. 3,616,015. In U.S. Pat. 3,616,176 the laminate is composed of a base sheet, with a polyamide layer covering the base sheet and a decorative ink layer covering the polyamide layer. Sufficient heat is applied to the laminate to heat the polyamide layer at or above its softening point, and the laminate is then pressed onto the surface of an article with the decorative ink layer coming into direct contact. Upon withdrawal of the heat source the polyamide layer cools to a temperature below its softening point and the base sheet is removed. The decorative layer becomes fused or heat sealed to the article. Since the polyamide layer lies over the decorative layer, it does not contact the article directly and therefore does not function as a contact adhesive. The decorative laminate disclosed in U.S. Pat. No. 3,616,176 has a significant disadvantage that since the base sheet is in contact with the laminate as it is imprinted onto the article, the laminate lacks sufficient flexibility to satisfactorily imprint surfaces having compound or sharp curvature.
U.S. patent application Ser. No. 130,303, commonly assigned with the present patent application, discloses a heat transfer label of the type illustrated in U.S. Pat. No. 3,616,015. The heat transfer label disclosed in Ser. No. 130,303 is composed of a carrier member (base sheet) overcoated in designated regions with a release layer and an ink design layer. Optionally a barrier layer is included between the release layer and the ink layer. The release layer is typically composed of a polymerization product of a diamine with the dimer of a fatty acid and is contoured to reduce the halo effect of the label as it is transferred onto an article. The optional barrier layer may be formed of an aromatic acid based polyester covering and overlapping the release layer by a margin. This patent application does not suggest a solution to the above-mentioned limitations of the heat transfer labelling process.
U.S. patent application Ser. No. 146,999 commonly assigned with the present patent application also discloses a heat transfer label of the type illustrated in U.S. Pat. No. 3,616,015. The heat transfer label disclosed in Ser. No. 146,999 is composed of a carrier member (base sheet) overcoated in designated regions with a release wax layer, a protective layer, an ink design layer and an adhesive layer. The protective layer provides enhanced chemical resistance for the heat transfer label and permits the heat transfer label to resist distortion during the heat transfer process. The protective layer is typically composed of an aromtic acid based polyester and a resin ester. This patent application does not suggest a solution to the above-mentioned limitations of the heat transfer labelling process.
A second prior art method for imprinting objects employs a flexible transfer pad. Ink is transferred to a transfer pad such as a porous silicone rubber pad, and then transferred directly from the pad to the surface of an article. This method has the advantage over the heat transfer labelling process disclosed in U.S. Pat. No. 3,616,015 in that it is suitable for imprinting articles having a wide range of shapes. This method has the additional advantage in that it employs assembly line equipment which is of simpler design than that disclosed in U.S. Pat. No. 3,616,015. Therefore there is less adjustment to the assembly line required in retooling the process to accept articles of a different size and shape. This feature allows smaller quantities of different sized articles to be imprinted with a reduction in the down time between runs. This process on the other hand suffers the disadvantage that only one color ink may be transferred at one time per transfer pad, thus making the process slow if multicolored designs are desired. Also since there is no protective coating covering the ink design, it is left exposed directly to the environment upon transfer to the article, thus directly subjecting the ink design to corrosive elements in the environment which would tend to deteriorate the print quality over a period of time.
U.S. Pat. No. 3,887,420 discloses the use of a silicone rubber pad to transfer designs from a decorative laminate to ceramic articles. The laminate includes a base layer such as a paper sheet overlayed with a coating of wax. The wax coating is coated with a film layer (Film B), which in turn is overcoated with an ink design layer and a second film (Film A). As the laminate is heated to within a narrow ten degree temperature range Film A is alleged to become adhesive while the wax coating and Film B become molten and nonadhesive. A flexible transfer pad, typically of silicone rubber, is pressed against the laminate to make contact with Film A. The transfer pad purportedly sticks to Film A so that as the transfer pad is withdrawn the substrate composed of the paper sheet and wax coating separates from the remainder of the laminate. The laminate adhering to the transfer pad is pressed onto a ceramic article, and the temperature of the laminate is dropped to within a narrow ten degree temperature range. At this temperature, Film A becomes adhesive and Film B is alleged to exhibit diminished adhesion. Thus, as the laminate is pressed onto the article with Film B contacting the article, the laminate is alleged to adhere to the article and released from the transfer pad as the pad is withdrawn.
The film layers A and B are each adhesive over only a very narrow ten degree temperature range, making it impracticable to control the described process within the context of an automated process, since each film layer must in turn be heated or cooled to within the required ten degree temperature range to make the process workable.
Precision heating or cooling of Film A and Film B to within such narrow temperature ranges is impossible to achieve or control within the split second time intervals required by an automated assembly process. Furthermore, the inclusion of a wax layer to form part of the substrate has the disadvantage that as the substrate is released from Film B there will be a tendency for a portion of the wax to remain attached to Film B. This will interfere with and retard the adhesive characteristic of Film B as the laminate is transferred from the transfer pad to an object.
Accordingly, it is an object of the present invention to provide a decorative laminate and method adaptable to an automated system for transferring the laminate from a support member to a transfer pad and thence to an article.
Another object of the invention is to provide a decorative laminate which achieves multicolor pad transfer decoration in a single transfer operation.
Another object of the invention is to provide a decorative laminate which permanently adheres to any article without subsequent firing of the laminate.
A further object of the invention is to achieve a pad transfer method which satisfies the above criteria while being compatible with automated operation.